Of all of America's largest urban areas, perhaps none represents the changing face of the nation better than Houston. During the last century it has grown from a minor regional outpost in the sultry southeast corner of Texas to a cosmopolitan hub of about 6 million people that is home not only to the U.S. energy industry but also to thousands of small businesses - about a third, by census estimates - that cater to and were started by the rapidly increasing minority population.

Immigrants, legal and otherwise, have altered significantly the makeup of the fourth-largest city in the country. A population that was largely white at the start of the 20th century and remained so 50 years later is today a modern melting pot, having doubled in size in the last 30 years. Decades of influx from all over the world have changed a typically biracial Southern city into one where there is no majority population: About 40 percent of Harris County residents are Hispanic, 33 percent white and 18 percent black. In 1960, the Asian population was negligible, but 50 years later it has grown to almost 8 percent.

Houston's dramatic growth is owed in part to its reputation as a magnet for economic opportunity. Even in the midst of a national economic slump, the city continued to thrive. Oil has helped, of course. The worldwide demand for energy resources rarely seems to wane as it once did. But Houston also boasts a far more diversified economy than ever before. Its port is the busiest in the nation if measured by foreign tonnage and its medical center is the world's largest - two engines that keep people coming and business humming. Its proximity to Mexico also has ensured a stream of Mexican and Central American immigrants, a controversial trend that nevertheless has added greatly to economic activity.

Rice University sociologist Stephen Klineberg has overseen the Houston Area Survey over the last three decades and tracked significant population, demographic and employment changes in that time. While noting the demographic "revolution" Houston has undergone, his most recent survey also includes a big caution sign for the future.

"The 'resource economy' of the industrial era, for which this city was so favorably positioned, has been replaced by a new high-technology, knowledge-based, fully worldwide marketplace," Klineberg's 2012 report states. "The traditional 'blue collar path' to financial security has now largely disappeared. Almost all the well-paid jobs today require high levels of technical skills and educational credentials."

Three decades of exploding growth, however, has at least assured ongoing employment of some sort, with an abundance of low-end service jobs helping keep unemployment levels consistently low and immigration constant. The city's pro-growth and pro-business policies mean that even parts of the city that have undergone radical ethnic change have remained vibrant. Strip centers that once catered to a younger collection of Anglo office workers, for example, are now chockablock with small businesses that serve recent Hispanic immigrant arrivals.

Many of the suburbs have undergone similar transformation. Fort Bend County is the most ethnically diverse in America, Klineberg said. And as predominantly white baby boomers age and then die off, much of the country will resemble Houston's western suburbs, where the white population has dropped from half of the total to about a third in the last 10 years, with the remainder divided almost evenly between Hispanic, black and Asian.

Yet another trend finds Houston getting younger as it ages. Its median age was about 33 in 2010, making it the second-youngest of the top 20 metropolitan areas in the U.S. The future of all the new residents looks as bright here as anywhere, though there is a growing concern about a new form of class segregation that has emerged even as the ethnic profile has changed. The Houston metro area is becoming more residentially segregated by income than most cities in the country, experts say. Plenty of vacant land and rapid growth have created large communities whose residents are as similar in income level as they are different in racial and ethnic composition.

The good news is that the median household income in the Houston area - $54,146 - is higher overall, almost 8 percent above the national average.